My husband has Parkinson’s and dementia.

When it is no longer safe for him to be home alone.
Unfortunately often we don't know it is not safe until something happens.
Is there a Senior Center where he can go for activities?
Is there an Adult Day Program that he can go to for activities?
Honestly if you are willing to have someone come in and he does not mind then I believe the sooner you get help the better. It might help you more than you realize. The caregiver can also be asked to do light housework, laundry and if you have hired someone with the expectation that they will be able to drive the caregiver can take your husband shopping,
You can check with your local Senior Service Center or Area Agency on Aging to find out if he/you qualify for any services.

When he’s not safe to be alone, which is likely now

I agree, if you are raising the topic without details , my guess its time to start it up.

It’s time when you start asking questions about when it’s time.